1. Field of the Invention
The present invention relates generally to clean pumps. More particularly, the present invention relates to clean pumps which are provided with an impeller held and rotated at a predetermined position in a casing by magnetic action from outside the casing and applicable to semiconductor manufacturing technologies and medical instruments.
2. Description of the Background Art
A centrifugal pump, in which pressure is increased through a rotary motion of fluid responsive to the rotation of an impeller, is widely used in the industrial field.
FIG. 1 is a vertical section of a clean pump proposed in Japanese Patent Laying-Open No. 3-229987 by the inventor of the present application. With reference to FIG. 1, an impeller 3 is provided in a casing 2 of a pump 1. Casing 2 is made of a non-magnetic member, while impeller 3 includes a non-magnetic member 5 having a permanent magnet 4 forming a passive magnetic bearing, and a soft iron member 6 equivalent to a rotor of an active magnetic bearing. Permanent magnet 4 is divided in the direction of the circumference of impeller 3, adjacent portions of which are magnetized in opposite poles to each other. A rotor 8 supported by a shaft 7 is provided outside casing 2 so as to be opposed to the side of permanent magnet 4 of impeller 3. Rotor 8 is driven to rotate by a motor (not shown). Rotor 8 is provided with the same number of permanent magnets 9 as that of permanent magnets 4 of impeller 3 so as to be opposed to the same and to cause an attracting force. Attached to be opposed to the side of iron member 6 of impeller 3 is an electromagnet 10 which acts to hold impeller 3 at the center of casing 2 against attracting forces of permanent magnets 4 and 9.
In thus structured clean pump, permanent magnet 9 buried in rotor 8 drives and radially supports impeller 3 to produce an axial attracting force between permanent magnet 9 and permanent magnet 5. Current is conducted through a coil of electromagnet 10 so as to maintain the balance between the attracting force, thereby floating impeller 3. Then, when rotor 8 is rotated, permanent magnets 4 and 9 form a magnetic coupling to rotate impeller 3, whereby fluid goes out from an outlet port (not shown). Since impeller 3 is separated from rotor 8 by casing 2 and will not be contaminated by electromagnet 10, the fluid from pump 1 remains clean.
The smaller the attracting force of the magnetic coupling between permanent magnets 4 and 9, the less amount of current flows through the coil of electromagnet 10 to generate less heat and consume less power. However, a reduced attracting force is not practical because it involves reduction of a driving force of the magnetic coupling and a radial rigidity. In order to increase a driving force required and a radial rigidity, current of more than a fixed amount should be conducted through the coil of electromagnet 10, resulting in an increase in heat generation and power consumption.